Heretofore, as an optical head device for writing optical information on or reading out optical information from optical discs and optical magnetic discs, one using a prism type beam splitter, and one using a diffraction grating or hologram element, as an optical part for introducing (beam splitting) a signal light reflected from the recording surface of a disc to a detecting section, have been known.
Heretofore, the diffraction grating or the hologram element for an optical head device has been prepared by forming a rectangular grating (relief) having a rectangular cross-section on a glass or plastic substrate by a dry etching method or an injection molding method, thereby to diffract light and provide a beam splitting function.
Further, in a case where it is attempted to improve the light utilization efficiency over an isotropic diffraction grating whereby the light utilization efficiency is about 10%, it is conceivable to utilize polarization. For utilizing polarization, there has been a method of increasing the go and return efficiency by increasing the efficiency of both going (in the direction from the light source to the recording surface) and returning (in the direction from the recording surface to the detecting section) by combining a quarter-wave sheet to a prism type beam splitter.
However, the prism type polarized beam splitter is expensive, and other systems have been sought. As one system, a method has been known in which a flat plate of berefringence crystal such as LiNbO.sub.3 is used, and an anisotropic diffraction grating is formed on its surface to provide deflection selectivity. However, the berefringence crystal itself is expensive, and it is difficult to apply such a method to the consumer product field.
With the isotropic diffraction grating, the utilization efficiency in going (in the direction from the light source to the recording surface) is about 50%, and the utilization efficiency in returning (in the direction from the recording surface to the detecting section) is 20%, as mentioned above, whereby the go and return utilization is at a level of 10% at best.
It is an object of the present invention to solve the above-described problems and to provide an optical head device which improves the light utilization efficiency and which can be prepared at a low cost with high productivity, and a process for its production.